1. The Field of the Invention
The present invention relates to a method and apparatus to determine the flow regime of at least a two-phase fluid, such as a gas-liquid mixture, flowing through a pipe and, in particular, to a method and apparatus which are passive in nature, relying substantially entirely upon the acoustics created by the passage of the fluid mixture through the conduit.
2. The Prior Art
When two or more different fluids flow together as a mixture in a single conduit, the pattern of separation between the fluids (the flow regime) is determined by a number of factors including the relative mass ratio of one fluid to the other, the velocity of each fluid (the difference in velocity of one fluid with respect to another fluid is referred to as the "slip velocity"), the difference in viscosity between the fluids etc. It is to be understood that the general term "fluid" is here used to include gases, liquids, and solids in powder or pellet form. Flow regimes are generally categorized by terms which graphically describe the interface between the fluids in the mixture. At low velocity the mixture will generally separate into layers within the pipe, according to the density of the fluids, with the fluid of the greatest density occupying the lowest portion of the cross section of the pipe and the fluid of the least density occupying the uppermost portion of the pipe's cross section. When the interfaces between the fluids approximate horizontal planes, the fluid is characterized as a "stratified smooth flow". Increasing the velocity, or altering other factors of the mixture, can cause the interfaces between the fluids to become rippled in similar fashion to the surface of a body of water. Flow of mixtures with rippled, but distinct and density dominated interfaces between the fluids, is called "stratified wavy flow".
In gas-liquid systems with very low gas flow rates, a flow regime will develop such that the gasses are contained as discrete bubbles throughout the liquid. This is known as a "disbursed bubble flow". With higher gas flow rates, or different fluid properties, the bubbles can coalesce into long gas pockets which generally travel at the velocity of the liquid. This regime is called "elongated bubble flow". At high gas flow rates and low fluid flow rates, the gas may create waves on the surface of the liquid with enough height to cause the pipe to become bridged forming a liquid slug. The liquid slug then travels with a velocity equal to the velocity of the gas which created it. This regime is obviously known as "slug flow". At very high ratios of gas to liquid, the liquid may form an annular film in contact with the entire inner surface of the pipe, while the gas flows through the center of the pipe and liquid film causing a condition known as an "annular flow". Sometimes the gas flow within the annular liquid film contains small droplets of the entrained liquid to create the condition known as "annular mist flow". In general the flow regime observed in a given fluid flow system is a function of the relative contribution of the gravitational forces and viscous forces acting on the fluids individually and on the interface between the fluids.
There have been previous attempts made to monitor two-phase fluids flowing in pipes. An example is found in U.S. Pat. No. 4,193,290 in which a portion of the two-phase steam is bled through an orifice to an acoustical transducer which generates an output signal having an amplitude proportional to the quality of the steam being monitored.
U.S. Pat. No. 4,683,759 shows a device which employs gamma radiation transmission by one-shot collimation to determine the distribution of voids within a gas-liquid mixture. The distribution of voids in selective portions of the pipe, taken together with statistical and logical tests applied thereto, provide information from which are determined: the type of flow pattern or flow regime, the profile of a large gas bubble in slug flow, and the gas-liquid volume flow rates in slug flow.
U.S. Pat. No. 5,148,405 concerns a non-intrusive method for determining the characteristics of slug flow in multiphase flow pipelines by detecting acoustic emissions from the pipeline in the ultrasonic frequency range by means of at least one sound transducer. The output of the transducer is an analog electrical signal which is converted to a digital signal and analyzed to determine the characteristics of the slug flow.
It is also know that "sand-pipe" interaction, namely the movement of sand particles entrained in a fluid flowing in a pipe, generates acoustical noise in frequency ranges of 300 kHz to 800 kHz and which can be detected. These high frequencies are probably due to the very high flow velocities of fluids in pipes as described in "A Simple Clamp-on Sand Detector For Subsea Installation" by Asle Lygre, Trond Folkestad, Chr. Michelson Institute, Norway "Petroleum Abstracts #542,944".